Gaseous fuels can be stored at cryogenic temperatures when employed as fuel for internal combustion engines. A gaseous fuel is defined as any fuel that is in a gas state at standard temperature and pressure which is defined herein as 1 atmosphere and between 20 and 25 degrees Celsius. The gaseous fuel is stored near its boiling point in a storage vessel. For example, for methane at a storage pressure of about 1 atmosphere it can be stored in liquefied form at a temperature of about −161 degrees Celsius. Natural gas is a mixture of gasses with methane typically comprising the largest fraction, storage temperature can vary, but is normally close to that of methane. From the storage vessel the liquefied gas is pumped in a liquid state towards and through a heat exchanger where it undergoes a transition to either the supercritical state or the gas state depending upon temperature and pressure of the gaseous fuel leaving the exchanger. There are advantages to storing the gaseous fuel in a liquefied state. The energy density increases when the gaseous fuel is in the liquid state compared to either the supercritical state or the gas state requiring a smaller volume to store an equivalent amount of fuel on an energy basis. Since liquids are relatively incompressible compared to gasses, it is more efficient to pressurize a gaseous fuel when in the liquid state compared to the either the supercritical or the gas state. After vaporization in the heat exchanger a fuel injection system receives vaporized gaseous fuel and introduces it, either directly or indirectly, to one or more combustion chambers in the engine. Vaporizing refers to converting a fluid in a liquid state into either a supercritical state or a gas state in this specification. While natural gas (LNG) is an exemplary gaseous fuel, which is employed in many high horse power (marine, mining, locomotive) and heavy duty (hauling) engine applications, other gaseous fuels are equally applicable to the technique described herein.
A heat source is required in the heat exchanger to increase the temperature of the gaseous fuel above its boiling point. Engine coolant from the water jacket of the internal combustion engine can be employed as the heat source. The engine coolant is routed through a separate path in the heat exchanger such that waste heat from combustion is transferred to the liquefied gaseous fuel from the storage vessel causing it to evaporate. By employing waste heat from the combustion process efficiency is improved compared to employing energy derived from the engine output, for example such as electrical energy from a generator driven by the engine.
It is important to control the temperature of the gaseous fuel discharged from the heat exchanger for a number of reasons. First, the gaseous fuel discharged from the heat exchanger is normally required to be in a particular state, for example the supercritical state. Second, the temperature must be above a predetermined minimum value such that components downstream from the heat exchanger are protected from excessively cold temperatures that may cause component failure. When the temperature of gaseous fuel downstream of the heat exchanger drops below the predetermined minimum value, or if it is predicted to drop below the predetermined minimum value, then the pump transferring gaseous fuel from the storage vessel to the heat exchanger must be suspended (stopped). Delivery of gaseous fuel to the fuel injection system stops when the pump stops and available fuel injection pressure decreases below the requisite level as the engine continues to consume fuel. As available fuel injection pressure decreases the engine can be designed to continue operation with a derated power output and then eventually stop, or go to a back-up secondary fuel. This situation is not desirable.
It is possible for the temperature of gaseous fuel discharged from the heat exchanger to decrease below the predetermined minimum value when the engine coolant is too cold, or when the residence time of the gaseous fuel inside the heat exchanger is too short, or due to a combination of these two reasons. During normal engine operating conditions engine coolant temperature is maintained between a predetermined range. However, engine coolant temperature can deviate from this range for a variety of reasons. One such reason is cold start of the engine when engine coolant temperature is equivalent or near to ambient temperature, which is much lower than engine coolant temperature during normal engine operating conditions. Excessively cold ambient temperatures may also cause engine coolant temperature to drop below the predetermined temperature range, or at least worsen cold start performance.
The volume of gaseous fuel inside the heat exchanger is normally less than the maximum displacement volume of the pump. During each pump stroke, the complete volume of gaseous fuel within the heat exchanger is discharged at its outlet in addition to an extra volume of gaseous fuel equal to the difference between the pump displacement volume and heat exchanger volume. Under normal engine operating conditions the temperature differential between engine coolant and the liquefied gaseous fuel inside the heat exchanger is sufficient to completely vaporize the gaseous fuel discharged from the heat exchanger. However, when the engine coolant is too cold the residence time of the extra volume of gaseous fuel inside the heat exchanger is insufficient to effect its vaporization.
One technique to increase residence time of the gaseous fuel inside the heat exchanger is to decrease pump speed. However, there is a corresponding decrease in the flow rate of gaseous fuel when pump speed is decreased, which can cause fuel pressure downstream of the heat exchanger to drop or cause unwanted fuel pressure fluctuations. Normally, the engine is not running at full load and the pump does not need to be stroking continuously without suspension. It is possible under these conditions to decrease pump speed to increase residence time of the gaseous fuel in the heat exchanger. However, in systems where the pump is directly driven from the engine it is not possible to change pump speed apart from a change in engine speed.
Canadian Patent No. 2,523,732, published Apr. 20, 2006 by Batenburg et al., hereinafter Batenburg, discloses a fluid delivery system and method that pumps a process fluid from a cryogenic storage vessel and delivers it to an end user as a pressurized gas. The technique comprises starting a pump and pumping the process fluid when the process fluid pressure is below a predetermined low pressure threshold and stopping the pump when the process fluid pressure is above a predetermined high pressure threshold. The process fluid is directed to a vaporizer where it is vaporized by heat from a heat exchange fluid. The process fluid temperature is measured downstream from the vaporizer and the pump is temporarily suspended when the process fluid temperature is below a predetermined threshold temperature, and restarted based on predefined enabling conditions.
There is a need for an improved technique that prevents the suspension of a pump during adverse engine operating conditions which cause the temperature of vaporized gaseous fuel to drop below a predetermined minimum value. The present method and apparatus provide a technique for improving operation of an internal combustion engine fuelled with a liquefied gaseous fuel.